Conventionally, as a gas sensor using an organic semiconductor, semiconductor gas sensors of the electric resistance type wherein comb like electrodes are formed on the surface of an organic semiconductor film and gas concentration is sensed using the fact that the dark conductivity of the organic semiconductor layer changes due to adsorption of gas have been generally used. As an example of such a gas sensor, NO2 sensors using a change in dark conductivity of a phthalocyanine-based thin film are known (See M. Passard, A. Pauly, J. P. Germain, C. Maleysson, Synthetic Metals, 80, 25 (1996), for example.)
On the other hand, the present invention is based on the photocurrent multiplication phenomenon occurring at an organic/metallic interface (See M. Hiramoto, T. Imahigashi, M. Yokoyama, Applied Physics Letters, 64, 187 (1994), for example, with regard to an n-type organic semiconductor, and M. Hiramoto, S. Kawase, M. Yokoyama, Jpn, J. Appl. Phys., 35, L349 (1996), for example, with regard to a p-type organic semiconductor.)
In the conventional gas sensor using an organic semiconductor as described above, change in dark conductivity due to adsorbed gas is used for sensing the adsorbed gas. However, since the dark conductivity of the organic semiconductor is extremely small, and the current is measured by means of comb like electrodes of which electrode-to-electrode distance is normally about 100 μm, an absolute value of the detected current is very small in the order of nA (nano ampere), leading the drawback that it is difficult to achieve accurate measurement.
In addition, since the change in dark conductivity is measured by measuring a change in resistance of entire organic thin film bulk, diffusion of molecules into the film bulk is a rate-determining factor, so that the drawback of slow response also arises.
In view of the above, it is an object of the present invention to provide a gas sensing method which improves the sensitivity of gas sensing and the response speed, and a gas sensor for implementing the method.